1. Field of the Invention
The present invention relates to unfrozen cold storage of perishable materials. More particularly, the present invention relates to packs, which incorporate thermal control material, capable of maintaining perishable biological materials that are in contact with or are enclosed by the packs at selectable temperatures, including sub-zero degree centigrade (xe2x80x9cC.xe2x80x9d) temperatures, for protracted periods of time.
2. Description of the Related Technology
It is known that perishable materials especially biological materials, e.g., food, flowers, pharmaceuticals, etc., can be stored at temperatures reduced from those at ambient to decrease rates of deterioration for extended periods of time. Reduced temperatures inhibit, for example, the activity of degradation enzymes indigenous to many biological materials as well as inhibit the growth of microorganisms. Currently practiced methodologies of reduced temperature storage can be divided into two categories: 1) storage of materials in an unfrozen state; and 2) storage of materials in a frozen state. When stored unfrozen, materials now generally are refrigerated or maintained at temperatures between 0xc2x0 C. and 10xc2x0 C. Alternatively, materials, when stored in a frozen state, customarily are stored at temperatures of xe2x88x9215xc2x0 C. or less. These practiced methodologies do not normally utilize the temperature range between about 0xc2x0 C. and xe2x88x9215xc2x0 C.
Existing cold storage methods and technologies suffer from serious defects. For example, with unfrozen storage, the temperature reductions into the 10xc2x0 C. to 0xc2x0 C. range slows enzymatic degradation and microbial growth in biological materials, but does not stop these processes completely. Thus, maintaining biological materials at temperatures between 0xc2x0 C. and 10xc2x0 C. will extend storage times, but such extensions are actually limited in duration from what is now known to be feasible, as is discussed below. Some biological materials stored at temperatures from 0xc2x0 C. and above, such as RNA and mixed pharmaceutical test reagents, begin to undergo a noticeable amount of deterioration in as short a period as one or two days, and can become completely unusable after two or more days. As is generally appreciated, short storage times place major constraints on the availability of fresh, non-frozen materials such as foodstuffs and/or other biological materials, such as vaccines and other biomedical materials. In essence, these materials must be obtained or produced in close proximity to where they will be sold or used in order to provide commercially practical storage times after shipping.
Freezing biological materials overcomes some of the deleterious consequences inherent in shipping fresh materials at unfrozen temperatures. For example, once frozen, biological materials may be stored for protracted periods during which they can be shipped over long distances, because freezing essentially stops enzymatic and microbial degradation processes. However, ice crystals unavoidably form within the biological materials during freezing, these crystals can damage the materials. Specifically, the formation of ice crystals can destroy the cellular integrity of the materials or cause xe2x80x9cfreezer burn.xe2x80x9d As is generally appreciated, the consequential damage to biological materials resulting from freezing reduces the quality of the thawed materials. In particular, with many foodstuffs, for example, the reduction in the quality caused by freezing results in reduced palatability and a corresponding reduction in the commercial value of the food relative to that same food in a fresh, unfrozen state.
In U.S. Pat. No. 5,804,444 (the xe2x80x9c""444 patentxe2x80x9d) to Kukal and Allen (the same inventors as here), which is hereby incorporated by reference in its entirety, the present inventors disclose novel technology for storing any biological material in an unfrozen state by determining the optimum storage temperatures for biological materials so as to overcome many of the limitations of prior storage methodologies. This novel technology is based on an appreciation of the fact that most biological materials have distinct sub-zero xc2x0 C. melting point depressions. By determining the melting point depression for a given biological material and then storing that material at its optimum storage temperature, which is slightly greater than but as close to the melting temperature as feasible, very substantial improvements in the duration and quality of the stored non-frozen biological material is achieved.
The discovery that biological materials have determinable lowest optimum storage temperatures at which they can be stored for extended periods of time has produced a need for refrigeration and packaging adapted to maintain biological materials at very stable temperatures just above the determined melting temperatures. These temperatures are predominantly below 0xc2x0 C.
Currently known and available cold storage packaging materials, such as those called xe2x80x9ccold packsxe2x80x9d or xe2x80x9cgel packs,xe2x80x9d are not capable of meeting these needs because they are not capable of being adjusted to maintain different specific temperatures required to achieve the improvements in storage of non-frozen biological materials described above. These known devices, i.e., xe2x80x9ccold packsxe2x80x9d or xe2x80x9cgel packsxe2x80x9d, for storing biological materials at reduced temperatures generally fall into two categories: 1) those that function by absorbing sensible heat to preserve storage temperatures reduced from ambient; and 2) those that function utilizing a latent heat capacitance incorporated in the container to preserve a desired storage temperature. As used here, xe2x80x9csensible heatxe2x80x9d is heat energy absorbed by a thermal control or temperature maintenance material that results in a corresponding increase in the temperature of the temperature maintenance material. Consequently, it is not feasible to maintain a desired temperature using a pack that functions by absorbing sensible heat, because as heat is absorbed the maintained temperature concomitantly increases. In contrast, xe2x80x9clatent heatxe2x80x9d is a determinable quantity of heat energy for a specified mass of a temperature maintenance material required to affect a phase transition in the material, e.g., from frozen to liquid states. During the phase transition the material maintains a substantially constant melting temperature. Latent heat equates to the amount of heat energy required to cause a given mass of solid material that is maintained at its melting temperature to become a liquid at that same temperature. Thus, the solid material continues to maintain a substantially constant temperaturexe2x80x94its melting temperaturexe2x80x94while external input heat energy is absorbed by the solid material to provide latent heat for affecting the process of melting.
An advantage in using latent heat when absorbing heat energy, as opposed to utilizing a sensible heat absorption process during corresponding warming, lies in the typically tremendous thermal capacitance associated with a material undergoing a phase change. In the case of water, one kilocalorie (1 Kcal) of heat absorbed as sensible heat is required to raise the temperature of one kilogram (1 kg) of the water by 1xc2x0 C. Whereas, the same 1 kg of water in a solid ice state at 0xc2x0 C. requires absorption of 144 Kcal of heat to affect the phase change to the liquid state for the water that remains at 0xc2x0 C. throughout the phase change. Hence, 144 Kcal of heat can be absorbed by the 1 kg of ice which maintains a constant temperature of 0xc2x0 C. In sum, utilization of latent heat permits absorption of increased amounts of heat energy while a melting temperature is maintained; whereas, absorption of sensible heat occurs over an unavoidable ever increasing dynamic range of temperatures.
Despite the advantages of using latent heat processes, there still are many drawbacks encountered when using currently available containers or packs employing latent heat to maintain constant temperatures. For example, these devices are not capable of providing selectable melting temperatures, and, thus, are not suitable for use in maintaining a range of biological materials at their distinct optimum storage temperatures. Furthermore, the current technology does not provide storage devices that are adapted for adjustment in order to change storage temperatures between storage uses. In other words, depending on what is being stored, different melting temperatures cannot be provided for different food products or other biological materials.
An example of such a prior device is the modular cold-storage pallet, including an insertable heat sink material utilizing latent heat, that is described in U.S. Pat. No. 6,266,972 to Bostic.
The foregoing underscores drawbacks and problems associated with conventional storage container and pack technology. In particular, the foregoing highlights problems associated with such devices, and their methods of use. Furthermore, the foregoing highlights the current, yet unresolved, need for storage containers capable of maintaining stable specific temperatures, in particular sub-0xc2x0 C. temperatures. Also, the foregoing highlights the need for storage containers and packs that can be used to maintain biological materials at their optimum storage temperatures as taught in the ""444 patent in unfrozen (fresh) states to maximize shelf lives and optimize quality.
The present invention overcomes the practical problems described above and offers new advantages as well.
The present invention is applicable to either thermal storage container or pack devices. As used here, thermal storage containers are intended to include any structure having curved sides, flat sides or a combination of all side shapes that can be in contact with or adjacent to stored materials placed in the containers. More specifically, containers usable with the present invention could be bowl shaped, open topped boxes, or closed topped boxes. Other configurations are, of course, feasible. Irrespective of the shape of a thermal storage container, the determinative aspect for any thermal storage container to be applicable to the present invention is that there be a capability to load a thermal control material such as a solution that can be frozen inside the structure of the container. Consequently, when heat is transferred from a stored material and its environment by conduction, convection, or radiation the transferred heat, or a significant portion of it, is absorbed by the frozen thermal control material to be utilized as latent heat in affecting melting of the frozen thermal control material. Generally, containers applicable to the present invention will have an interior compartment or compartments in which a thermal control material can be loaded. These compartments usually contain the thermal control materials so that they are not in direct contact with stored biological materials. In contrast, packs that are applicable to the present invention, in general, can be bag shaped devices which can be filled with a thermal control material such as a solution that can be frozen, or can be essentially rigid shaped devices having curved, flat, or a combination of such exterior surfaces. Again, the determinative aspect for a pack to be applicable to the present invention is that there be a capability to load a thermal control material that later can be frozen inside the pack. Hence, when heat is transferred from a stored material and its environment by conduction, convection, or radiation the transferred heat, or a significant portion of it, is absorbed by the frozen thermal control material to be utilized as latent heat in the melting process for the frozen thermal control material.
The present invention will be described below in the contexts of packs, but the descriptions are as applicable to container devices as described above and as such those containers are usable with and are within the scope of the present invention. These included containers, for example, can be multi-walled structures for storing processed or preprepared meals or food dishes, sometimes known of as Home Meal Replacements (xe2x80x9cHMRxe2x80x9d). Such containers can be used with thermal control materials of the present invention and after storage can be used as containers for heating or even serving the food. Additionally, the present invention will be described in the contexts of solutions for the thermal control materials, but any material capable of having selectable melting temperatures set is usable with and is within the scope of the present invention.
The solutions that are to be loaded into packs for the present invention are tailored to have specific melting temperatures selected to be the best temperatures for achieving extended storage times for materials to be stored statically or transported. As is discussed below such solutions can be solvent-solute, solvent-solvent, or other types of solutions that have had their melting points accurately tailored to preselected values, which are chosen to maximize storage times.
After a pack is loaded with a tailored solution the pack is subjected to below freezing temperatures to freeze the solution, and then the pack is allowed to thermally stabilize at the solution melting temperature for use in maintaining a stored material at its optimum storage temperature.
It is an object of the present invention to provide thermal control materials that melt at preselected temperatures that are optimum storage temperatures for materials being stored near the thermal control materials.
Another object of the present invention is to provide a storage container or pack filled with a thermal control material that has had its melting temperature tailored to a specific optimum storage temperature for a material being stored in the container or near the pack.
Yet another object of the present invention is to provide for refilling a storage container or pack with a changed thermal control material having an altered melting temperature so that a different material can be stored at its optimum storage temperature.